There are two major divisions into which pumps may be classified; Non-positive displacement and Positive displacement.
Non-positive displacement pumps induce flow by means of a device such as a propeller or centrifugal impeller. Such pumps are well suited for bulk transport but their flow output is difficult to predict as the flow output of non-positive displacement pumps is substantially dependent on fluid and system parameters such as discharge pressure and viscosity.
Positive displacement pumps move fluids by means of a displacement element sealed to retard or altogether prevent fluid leakage. A motive force applied to the displacement element regulates the flow rate. There are several types of such displacement elements or liquid ends, including piston, disc diaphragm, and tubular diaphragm. The liquid output of a positive displacement pump is accurate and predictable over a wide range of conditions, being governed only by the stroke rate and displacement volume of the piston.
The simplest such type is a piston sealed within a cylinder by a packing or sealing ring. Liquid, an incompressible fluid, contained between the housing and piston will discharge from the pump at a rate governed by the rate of the motive force resulting from advancement of the piston into the cylinder. Fluid is delivered at this rate in a manner essentially independent of fluid or discharge conditions.
"Milton Roy Meter Pump Technology," Bulletin 210, page 7 and 8 discloses a variety of disc diaphragm liquid ends, which builds upon the piston pump previously described. These disc diaphragm liquid ends have a diaphragm which acts as a barrier between the piston and the process fluid. The piston's pumping motion is applied to hydraulic fluid which communicates the piston action to the diaphragm and causes the diaphragm to flex back and forth as the piston reciprocates. The diaphragm, in turn acts upon the process fluid.
Typically a disc diaphragm liquid end uses discharge and suction contour plates to restrain the maximum suction and discharge position of the disk diaphragm. The volume contained between the contour plates minus the volume occupied by the diaphragm is generally set to be slightly greater than the volume displaced by the piston at full stroke. The diaphragm thereby oscillates between the confines of the contour plates without contacting said contour plates during normal pump operation. Slight piston leakage over time or other physical changes such as temperature induced expansion of working fluid or starved suction operation may cause the diaphragm to contact the suction or discharge contour plates.
Contact with the discharge contour plate, prior to completion of the piston discharge stroke, causes a pressure increase in the working fluid. The pressure increase is a result of the constraint on the discharge motion by the contour plate while the piston continues to force the diaphragm in the discharge direction. This pressure increase causes a relief valve to open, effectively bypassing the piston working fluid back to the supply reservoir. In a similar manner, contact with the suction contour plate prior to completion of a suction stroke causes a decrease in working fluid pressure opening the vacuum breaker relief valve. Contour plates serve to both describe the maximum allowable volume delivered per stroke and to keep the diaphragm synchronized with the piston.
It is possible to replace the piston and working fluid with an external source of air pressure and vacuum and a suitable valving arrangement. Here air pressure is directed to the diaphragm to effect a discharge stroke. The valve is switched to vent the air pressure and direct a vacuum to the diaphragm initiating a suction stroke. Here the delivered volume is equal to the volume captured between the contour plates minus the thickness of the diaphragm so long as the valve is maintained in pressure or vacuum for sufficient time to assure full contact with each respective plate.
"Milton Roy Metering Pump Technology," Bulletin 210, page 7 discloses a tubular diaphragm liquid end, which places two different flexible, sealed members between the piston and the process fluid. In this liquid end, the piston forces hydraulic fluid to flex a disc diaphragm. The disc diaphragm then transfers the pumping action to an intermediate fluid which surrounds an elastomeric tubular diaphragm, through which the process fluid passes. A tubular diaphragm liquid end has been primarily used for pumping slurries and viscous applications.
These types of positive displacement pumps use a piston, and the more advanced liquid ends also use some sort of diaphragm. The flow rates for such positive displacement pumps are generally adjusted by modulating the stroking rate and the stroke length of the piston. Two patents which disclose methods used to adjust the stroke length of a displacement element are U.S. Pat. No. 2,856,857 to Salfrank and U.S. Pat. No. 3,769,879 to Loftquist. A variable speed motor is a common method of adjusting stroking rate. In a diaphragm type liquid end, the stroke length has also been modified by bypassing the working fluid for a portion of the piston stroke to remove motive force from the diaphragm. Bypassing is an effective and inexpensive stroke adjustment means but imposes shock on the mechanism and process fluid plumbing. These mechanical stroke adjustments tend to be complex, expensive and difficult to economically miniaturize.